1. Field of the Invention
This invention relates to bioreactors, and in particular to an integrated synchronous aerobic/anaerobic bioreactor.
Interest in anaerobic biotechnology for industrial wastewater treatment has greatly increased during the past decade, because the anaerobic process is the most elegant method to reduce carbon pollution. With minimum power requirement, anaerobic technology converts 90% of the pollutants into a valuable form (Verstraete et al., 1990).sup.1. In contrast, aerobic processes transform 50% of the carbon pollution into surplus sludge which still requires other processes to deal with. However, anaerobic technology has inherent limitations. Methanogens, which are at the last stage of the anaerobic process chain, have a limited substrate affinity. As a result, anaerobic systems are inefficient in treatment polishing. In comparison, aerobic treatment permits the removal of trace organics with, in practice, a capacity of purification down to values lower than the required standards (&lt;30 mg BOD L.sup.-1) (Vochten et al., 1988).sup.2. Furthermore, aerobic processes are recognized to have the capability to mineralize a broader range of recalcitrant compounds than anaerobic processes. For example, common compounds refractory to anaerobic treatment include some chlorinated phenolics, vinyl chlorides, resin acids, terpenes.
2. Description of the Prior Art
In all these cases where anaerobic treatment is employed, an aerobic process is also included for secondary or tertiary treatment. The objective of sequential anaerobic-aerobic system is to maximize the abatement of the chemical oxygen demand (COD) and to release an effluent that is not acutely toxic.
In most of the above cases, anaerobic transformation is thus incomplete, and less chlorinated aliphatics or aromatics are the endproducts of the anaerobic processes. In contrast, aerobic microorganisms are efficient degraders of less chlorinated organic compounds up to complete mineralization. Anaerobic digestion is thus indicated as a primary treatment step to convey less chlorinated or dechlorinated compounds-containing effluents to an aerobic polishing unit.
In such sequential treatment schemes, the anaerobic and aerobic bacteria operate in separate units that complement each other. Despite its great potential, the anaerobic/aerobic sequence has drawbacks. In some cases, anaerobic partial degradation results in products which are just as or more toxic than the primary molecule. These products can accumulate in the anaerobic stage where they could inhibit anaerobic microorganisms themselves prior to being released into the subsequent aerobic unit. This might decrease the effectiveness of the overall system from a certain feeding level.
Recently, attention has been directed to the use of a single combined anaerobic/aerobic system. For example, it is known that aerobic and anaerobic microorganisms can grow in the same habitat provided that the input of O.sub.2 is lower than the potential rate of consumption, which causes O.sub.2 -limited environments. This is typical of biofilm systems. Limitation in the molecular diffusion of O.sub.2 results in abrupt O.sub.2 concentration downward gradients, leaving a large portion of the biofilm volume free of O.sub.2 (Table 1). In all cases shown, over 63% of the total biomass is anaerobic.
TABLE 1 ______________________________________ LIMITATION IN O.sub.2 MASS TRANSPER IN VARIOUS TYPES OF BIOFILMS Trickling Mycelial Enterobacter cloacae System Filter Slime.sup.a pellet.sup.b in alginate bead.sup.c ______________________________________ Dissolved O.sub.2 in 8.5 7.8 7.4 bulk liquid (ppm) Biofilm thickness 0.4-1 3 1.5 or radius (mm) Depth of O.sub.2 150 135 150 penetration (.mu.m) Relative volume 63-85 87 73 free of O.sub.2 (%) ______________________________________ .sup.a Chen and Bungay, 1981.sup.3 ; .sup.b Huang and Bungay, 1973.sup.4 ; .sup.c Beunink et al., 1989.sup.5.
This disadvantage of biofilms was exploited to develop a co-culture of a strict aerobe (Alcaligenes) with a facultative anaerobe (Enterobacter cloacae), both immobilized within Ca-alginate beads of ca. 3 mm of diameter (Beunink and Rehm, 1990).sup.6. Both microorganisms which were initially distributed homogenously within the bead matrix, were rapidly shared out differentially amongst the inner and the outer space of the bead, due to the selective pressure exerted by the oxygen in the outer space, and its drastic limitation in the inner space. The authors showed that only this synergistic arrangement was able to completely degrade the 4-chloro-2-nitrophenol (CNP). Otherwise CNP is totally refractory to pure aerobic cultures of Alcaligenes alone, while in the presence of a pure anaerobic culture of Enterobacter alone, CNP transformation was limited to the 4-chloro-2-aminophenol. The nitro-group first had to be anaerobically reduced before dioxygenases of Alcaligenes were able to cleave the aromatic ring and proceed to its mineralization. Even though the application of this laboratory system at a large scale is questionable (use of pure strains; oxygen tolerance of the anaerobic species which is facultative; use of alginate beads as immobilization carriers), it represents an excellent model which demonstrates the interest of coupling reductive and oxidative catabolisms.
Other examples of mixed anaerobic/aerobic cultures are described in Gerriste et al 1990.sup.7.